Intake and/or exhaust gas sensors may be operated to provide indications of various exhaust gas constituents. For example, an oxygen sensor positioned in an engine exhaust system may be used to determine the air-fuel ratio (AFR) of exhaust gas, while an oxygen sensor positioned in an engine intake system may be used to determine the concentration of exhaust gas recirculation (EGR) gasses in intake charge air. Both parameters, among others that may be measured via an oxygen sensor, may be used to adjust various aspects of engine operation. An engine may be controlled to achieve a desired exhaust gas AFR based on the AFR indicated by an oxygen sensor to maximize operating efficiency of an emission control device, for example. For some oxygen sensors, their output may significantly vary as a function of their operating temperature. Accordingly, such oxygen sensors may be heated by a heating element to bring the sensor temperature within a desired range such that desired oxygen sensing is provided. In some examples, the heating element is controlled according to a desired temperature and an inferred temperature, which may be determined based on the resistance of the heating element, as the heater resistance may vary linearly with heater temperature. The resistance-temperature heater transfer function may differ among different oxygen sensors, however.
U.S. Pat. App. No. 2003/0019865 discloses methods of controlling a heating element of an exhaust gas oxygen sensor. Particularly, disparities in the resistance-temperature heater transfer function among oxygen sensors due to sensor-to-sensor variability are recognized and addressed by employing an adaptive offset (e.g., forming, with other parameters, a y-intercept) in a linear function relating heating element temperature to heating element resistance. The offset is adjusted based on a deviation between a measured heating element resistance from its nominal value under predetermined conditions at engine start-up. The linear function includes a slope relating heater resistance to heater temperature that is manufacturer-specified.
The inventors herein have recognized an issue with the approach identified above. Variance in the resistance-temperature transfer function of an oxygen sensor may include variance in both the offset and slope of a linear function used to determine heating element temperature as a function of heating element resistance. Being manufacturer-specified, the slope of the above-identified approach is not adapted throughout the life of an oxygen sensor, which may lead to inaccurate oxygen sensor control that can in turn cause increased emissions, decreased fuel economy, and decreased vehicle drivability.
Oxygen sensors may exhibit additional variability that can affect sensor and heating element control. In some approaches, the temperature of an oxygen sensor is controlled based on the impedance of a sensor element (e.g., a Nernst concentration cell) of the sensor; as the sensor element impedance may be a function of temperature, the sensor temperature may be controlled to a desired temperature by bringing the sensor element impedance to a desired impedance. The relation between sensor element impedance and temperature often varies among oxygen sensors, however, and with age.
U.S. Pat. No. 5,852,228 discloses methods and apparatuses for achieving a target sensor element impedance so as to bring an oxygen sensor to a desired temperature. The increase of sensor element impedance with sensor element deterioration is recognized and addressed by altering the target impedance as a function of the power supplied to the sensor heating element. Specifically, one of four target impedances may be selected depending on the average power supplied to the heating element. A transition from a relatively lower target impedance to a relatively higher target impedance may be achieved by incrementing the relatively lower impedance by a predetermined amount.
The inventors herein have recognized an issue with such an approach. In some scenarios, controlling an oxygen sensor based on a target impedance selected from four target impedances may result in an undesired sensor temperature that fails to enable desired sensor operation and/or can potentially degrade sensor operation, due to the lack of granularity of the selectable impedances. This issue is exacerbated by sensor aging, which may cause variation in the relation between impedance and temperature in the sensor.
One approach that addresses at least some of the above-identified issues includes a method of operating an oxygen sensor including a heater comprising sampling a first heater resistance at a first temperature, and determining a resistance-temperature transfer function relating heater resistance to heater temperature based on the first heater resistance and a second heater resistance at a second temperature, the second temperature different from the first temperature.
Another approach that addresses at least some of the above-identified issues includes a method of controlling an oxygen sensor comprising, responsive to determining that a temperature of the oxygen sensor corresponds to a desired temperature, determining an impedance of the oxygen sensor, setting an impedance setpoint to the determined impedance, and controlling the oxygen sensor so that the impedance of the oxygen sensor corresponds to the impedance setpoint.
In this way, the temperature of an oxygen sensor may be accurately controlled throughout its operational life, enabling increased accuracy of output from the oxygen sensor and parameters derived therefrom. Thus, the technical result is achieved by these actions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Finally, the above explanation does not admit any of the information or problems were well known.